--- a/www/HACKING.rst	Tue Nov 08 19:01:27 2016 +0200
+++ b/www/HACKING.rst	Wed Nov 09 03:16:32 2016 +0200
@@ -150,6 +150,16 @@
 * ``Br`` - Great Britain
 * ``Au`` - Australian
 
+``rare`` attribute to first headword used as marker that word has low frequency.
+SRS file writers skip entries marked as ``rare``. I found it convenient to check
+frequency with:
+
+https://books.google.com/ngrams/
+  Google N-grams from books 1800-2010.
+
+For cut-off point I chose ``beseech`` word. All less frequent words receive
+``rare`` marker.
+
 C5 dictionary source file format
 ================================